1. Field of the Invention
The present invention relates generally to short wavelength radiation, and more specifically to the generation and transportation of short wavelength radiation.
2. Technical Background
Short wavelength radiation finds wide application a diverse array of technologies. For example, the semiconductor industry uses ultraviolet radiation in photolithographic processes to define the conductive paths in integrated circuits. The lower limit of feature size is directly related to the wavelength of the radiation; hence, to form smaller features, it is desirable to use shorter wavelength radiation. Likewise, in metrology applications it is desirable to use radiation having as short a wavelength as possible to maximize resolution. Short wavelength radiation also finds use in medical and industrial applications.
While short wavelength radiation is extremely useful, there exist few workable methods to transmit it from one location (i.e. the source) to another (i.e. an instrument or a workpiece). Lenses and mirrors may be used to reflect and focus the radiation; such apparati are difficult to align, sensitive to vibrations, and may cause exposure of personnel to the radiation. Conventional optical fibers are generally not appropriate for transporting ultraviolet radiation (i.e. radiation with wavelength less than 400 nm), as their transmission of light with wavelengths less than about 370 nm is generally to be quite limited. For example, conventional germanium-doped silica waveguides tend to be photodarkened by 355 nm radiation. 248 nm radiation from an excimer source has reasonable transmission in conventional fibers (Ge-doped silica core/silica clad or silica core/F-doped silica clad) over only a very short distance (e.g. 0.5 m). Transmission of pulsed 248 nm radiation causes these fibers to develop color centers, further limiting their usefulness. Polymer waveguides are also unsuitable for transmission of ultraviolet radiation due to high loss and material damage. Short wavelength radiation is often used at high powers; conventional optical fibers are susceptible to damage or spurious effects in transmitting high power short wavelength radiation.
Hollow waveguides have been formed by coating the interior surface of 1 mm bore capillaries with reflecting layers of metal or metal/polymer. These have been used to transmit ultraviolet radiation down to 157 nm in wavelength with losses below 1 dB/m when straight; however, the losses increase to 2-4 dB/m when the waveguide is bent with a radius of 30 cm. Further, these hollow core waveguides are only slightly flexible, making routing short wavelength radiation around corners problematic. There exists a need for more efficient, safe, and flexible methods for transporting short wavelength radiation from one location to another.
In photolithography and metrology applications, it is desirable to use the shortest wavelength possible for which stability, spectral intensity, and beam quality requirements are satisfied. Currently, excimer lasers are widely used in these applications. Excimer lasers, while quite intense, suffer from relatively poor beam quality and are noisy with respect to intensity fluctuations. Further, excimer lasers are not easily tunable by an operator. Harmonic generation techniques have recently been identified as having the potential to deliver improved quality and to allow tunability of wavelength. However, current harmonic generation techniques tend to be quite low in output intensity. There remains a need for ultraviolet radiation sources having acceptable intensity, wavelength tunability over a wide range, and high beam quality.